Anthrax is a highly lethal infectious disease caused by the spore-forming bacterium Bacillus anthracis. The deliberate distribution of anthrax spores through US mail system in 2001 resulted in 5 deaths among the 11 individuals who contracted inhalational anthrax, which highlight the great threat posed by the potential use of anthrax in terrorism and warfare. The lethality of inhalational anthrax is primarily due to the action of anthrax toxins. Bacterium produces three toxin components; they are protective antigen (PA), lethal factor (LF), and edema factor (EF). PA together with LF forms lethal toxin (LT) and PA together with EF forms edema toxin (ET). PA functions as a vehicle to mediate the cellular uptake of the LF and EF. LF is a zinc-dependent endopeptidase that cleaves mitogen-activated protein kinase kinases (MEKs) and can replicate symptoms of anthrax disease when injected in animals with PA. EF is a calcium-calmodulin-dependent adenylate cyclase with a range of toxic effects in the host. These toxins are the dominant virulence factors for anthrax disease.
Currently there are no approved therapies for anthrax disease except antibiotics. Treatment with antibiotics, though, has considerable limitations. Exposure to the bacterium followed by bacterial division leads to the production of large quantities of the anthrax toxins. Thus, unless exposure is diagnosed early enough for vigorous antibiotic treatment, patients will succumb to disease even after the killing of all bacteria. The current vaccine approved by US Food and Drug Administration is also not effective in protecting newly infected individuals, as it requires repeated administration and at least 4 weeks for development of protective titers. Thus, there is a need for anthrax therapies that immediate neutralize the effects of one or more toxin components.